[filesystem] Greatly improve CBaseFileSystem::FixUpPath

[RaphaelIT7]

I've used this quite a bit to learn and find ways to optimize or solve things, so why not contribute to the main project :^

Changes:

• Geartly improve CBaseFileSystem::FixUpPath making the filesystem generally faster.

I also made a pr in the source-physics repo to solve a ivp bug that broke the entire physics engine.
Source-Authors/source-physics#1

[RaphaelIT7]

Added two more changes:
- Fixed a downcast issue that could possibly cause physics issues in the future
This was a bug that was found in gmod & fixed there, if the MAX_MAP_BRUSHSIDES was raised, any brush that had brush sides above the limit would have no collisions.
So this would only be a issue if you were to raise it.

• optimized CCvar::FindCommandBase
  I made this optimization today and didn't find any issues, so it should work fine.
  It's quiet a significant improvement but I don't got exact numbers, but from what I experienced it should be like 10x?
  The improvement should become bigger with each new command/convar being added.

Split into prs

[dimhotepus]

Thank you for all these changes!

I find a lot of them handy and will land into main when finish basic HL2 Anniversary Update content support (will ask you to rebase).

Could you please split this PR per commit so we can discuss, validate, merge (and rollback) every fix alone? It is easy to miss something when merging a lot of changes over entire codebase.

[RaphaelIT7]

Every change should how have it's own pr :D

[RaphaelIT7]

@dimhotepus I'm curious to know the state of all of these PRs, would it be possible to get some feedback to them? They've been sitting here for quite a while & I plan to soon start making more xd